Integrated circuit packaging has developed to the point that integrated circuits or microcircuits may be hermetically sealed into a package called a "flat pack" which comprises a base secured to a rectangular ring, with the microcircuit located on the base within the ring. The sides of the ring are provided with glass feedthroughs which are utilized to insulate and seal leads passing therethrough to the interior of the ring. In some instances, the interior ends of the leads form contact pads which are bonded to corresponding pads on the microcircuits so that electrical connection can be made to and from the microcircuit. In other instances, conventional wire bonding is utilized.
The height or profile of the so-called flat packs is in major part determined by the spacing between the topmost portion of the glass feedthrough and the lid or cover for the flat pack. In general, for a 40 mil wall thickness ring, the lid, if it is to be welded to the ring, must be at least 40 thousandths of an inch from the top of the glass feedthrough to prevent damage to the feedthrough due to the heat generated during welding. For thinner walls, such as for instance 30 thousandths of an inch, successful damage free welding requires that the lid be at least 50 thousandths of an inch from the glass feedthrough. It will therefore be appreciated that the package height is dependent upon a required spacing when welding techniques are utilized to secure the lid to the package.
Welding is desirable both because it is inexpensive and because the hermeticity achievable with welded lids far exceeds that of solder-based systems. Additionally, processing yields utilizing welding are considerably higher than the yields associated with soldering.
While nonweldable packages may have their respective heights or profiles reduced through the utilization of lids soldered to respective rings, soldering is both expensive in terms of process time and also in terms of the coating materials necessary for the soldering process. For instance, to achieve hermetic seals with standard processing techniques, both the package and the lid must be flash coated with gold, obviously an expensive metal. Nickel, on the other hand, is considerably less expensive but is not suitable for soldered flat packs.
While U.S. Pat. Nos. 4,331,258; 4,331,253; 4,262,300; 4,227,036; 4,192,433; 4,149,910; 4,135,038; 3,945,557; 3,874,549; 3,848,077; 3,823,468; 3,774,232; 3,768,144; 3,676,569; 3,669,715; 3,663,868; 3,648,357; 3,548,076; 3,538,597; 3,435,516; 3,423,638; and 3,349,481 all illustrate methods and apparatus for sealing flat packs, none of the above patents teaches limiting the height of a flat pack with a welded lid, nor means for preventing damage to glass feedthroughs when utilizing conventional welding.
Reference is made to U.S. Pat. No. 3,539,704 which shows the utilization of an annular lip of a ring sidewall to which is welded a bottom or cover. This patent is, however, directed to leadthrough assemblies having a plurality of passages in the leadthrough panel which extend at a 45.degree. angle to the plane of the enclosure, with the leadthrough panel being further provided with a stress decoupling area around the flange portion. It will be appreciated that the flange is integral to the ring of the package and is flexible at the bend for stress relief purposes. Because of the 45.degree. angle at which the leads enter the package, this patent is not concerned with minimizing package height, nor with the removal of heat from the leadthrough area. The 45.degree. angle of the leads is primarily used to reduce the stresses on the leadthrough assembly by reducing the amount of bending required on the conductors for connecting them to the adjoining circuitry. It can thus be seen that mechanical stress relief is what is covered by the above patent. Moreover, the stress relief is provided for the specific purpose of relieving the stresses normally attending a "cold weld" process. "Cold welding" does not provide the damaging heat of the conventional welding process. Neither does it provide for the required hermeticity or yields for flat packages.